It is well recognized that the successful dispersion and exfoliation of nanoparticles, like hydrous metal hydroxides (e.g., hydrotalcite) and clays (e.g., montmorillonite, saponite, hectorite, mica), requires surface treatment of the filler to promote wetting by hydrophobic polymers. For example, one common surface treatment applied to smectite clays, which dates back to the 1940s, involves ion exchange reactions between the basal surface of the clay and quaternary amines, such as dimethyl dihydrogenated tallow ammonium chloride and methyl benzyl dihydrogenated tallow ammonium chloride.
However, the prior art methods of forming polymer nanocomposites have failed to produce polymer nanocomposites that exhibit the expected performance characteristics suggesting that such methods are fundamentally flawed. For example, it is well known that the barrier properties of a polymer may be improved by forming a polymer-phyllosilicate nanocomposite. The mechanism responsible for barrier improvement is believed to involve the generation of a torturous diffusion path in which diffusing species must navigate a long and torturous path around the impermeable phyllosilicate platelets dispersed in the polymer matrix. Under the ideal conditions of complete phyllosilicate nanoparticle exfoliation and perfect platelet alignment within the polymer matrix, the reduction in gas permeability will be a function of the square of the phyllosilicate loading and the inverse square of the platelet aspect ratio. Accordingly, the steady-state gas permeability of the nanocomposite is expected to be reduced by as much as several hundred fold relative to the gas-permeability of the polymer with only 5-10 vol % nanoparticle loading. However, the barrier improvements in various polyolefin nanocomposite systems have been reported to be only two to four-fold at best suggesting that exfoliation of a surface-modified filler (in this example, the organophyllosilicate), according to prior art methods, is flawed in producing polymer nanocomposites, as evidenced by the significant variance in the barrier performance of prior art polymer nanocomposites from the expected barrier performance. The reasons for this deficiency in the prior art methods are undoubtedly multi-fold and complex, but it is clear that there is a need for methods of forming polymer nanocomposites which obviate the deficiencies of the prior art.
Some reasons for the deficiencies of the prior art appear to be (1) the failure of the prior art to appreciate the relationship between the spatial distribution of charge within the crystal lattice of a filler (e.g., clay) and self-assembled surfactant structures; and (2) the failure of the prior art to appreciate the correlation between the surface energy (i.e., critical surface tension) of the surface-modified filler surface and wetting by the polymer.
Other than the effects of the ion exchange capacity (e.g., CEC as it relates to the exchange of cations) of the filler on surfactant chain conformation, the prior art has failed to appreciate the relationship between the spatial distribution of charge within the crystal lattice of the filler and self-assembled surfactant structures. The prior art appears to be founded on the assumption that the charge centers within the filler are homogeneously distributed, and that the ion charge centers within in the interlayer space are also homogeneously distributed. These assumptions appear to persist despite the fact that no scientific evidence has been forthcoming to conclusively support these assumptions.
As it relates to the formation of organoclay-polymer nanocomposites, the prior art has failed to appreciate the presence of cationic charge segregation and its effect on organoclay properties. According to the present disclosure, it has been discovered that the existence of a two-dimensional amphiphilic surface morphology, resulting from cationic charge segregation, may explain why melt intercalation using organoclays of the prior art have heretofore failed to produce nanocomposites with polyolefin homopolymers. The state of cationic charge segregation is consistent with observations that water is rapidly adsorbed by organoclays prepared by prior art methods. Furthermore, Vaia and Giannelis in Macromolecules, 30, 8000-8009 (1997) ('97) conclude that the existence of polar interactions are a critical prerequisite for the formation of intercalated and exfoliated nanocomposites by melt intercalation.
The existence of a two-dimensional, amphiphilic morphology is further supported by the fact that organoclays of prior art methods are known to those skilled in the art to spread and form a monolayer at air/water interfaces. However, the significance of this spreading behavior in relation to the production of clay/polymer nanocomposites has not heretofore been recognized. According to the present disclosure, a surface-modified filler (e.g., organoclay) surface is more appropriately characterized by a surface hydrophilic lipophilic balance (HLB), rather than a solubility parameter as disclosed in U.S. Pat. No. 6,271,297 B1. Furthermore, the exfoliation of lamellar liquid crystal systems (e.g., organoclays), in organic solvents, is known to be limited by entropic effects, rather than effects relating to solubility parameters. In other words, the phase equilibrium (i.e., the tie line) existing between a lamellar liquid crystal phase and a continuous organic phase has never been shown to be dependent upon the solubility parameter of the organic phase. Consequently, solubility and cohesion parameters can provide no fundamental insight to the understanding of nanocomposite formation. While the utility of the surface HLB value has been discussed in prior art (see for example U.S. Pat. No. 7,160,942 B2), no method has been forthcoming to measure such values, nor has any prior art recognized the interrelation between solid surface energy, the surface HLB value, and the ability to produce clay/polymer nanocomposites by either melt intercalation or melt compounding of polyolefin homopolymers.
While the use of interfacial tension forces to estimate the nature and extent of interaction energies has been described in prior art (see Vaia and Giannelis ('97)), the approach does not contemplate nor demonstrate the effects of cation segregation on surface energy. Prior art methods for estimating interaction energies are based on van Oss theory, which has been used to define required polymer characteristics for producing an exfoliated nanocomposite system. In accordance with van Oss theory, prior art methods have emphasized organoclay designs that maximize the number of possible interaction sites between the polymer and the interlayer surface (i.e., bare clay surface deep within the palisade layer). However, according to the present disclosure, it has been discovered that the key to developing self-dispersing organoclays for polymers such as polyolefin homopolymers is to make the organoclay surface ‘organic-like’ as much as possible and to minimize or completely eliminate interactions between the polymer and the interlayer surface.
As is known, wetting of the organoclay surface by the polymer is a prerequisite for nanocomposite formation. However, the prior art has failed to appreciate the correlation between the surface energy (i.e., critical surface tension) of the surface-modified filler (e.g., the organoclay) surface with polymer wetting. It appears that the prior art has assumed that the contact angles at the organoclay/polymer interface are zero, despite the lack of scientific evidence in support of this assumption.
Accordingly, there is a need for the design and formation of new surface-modified fillers that enable significant improvements in the ease of dispersion of the nanoparticles, and which provide polymer composites and nanocomposites that demonstrate significant improvements in physical properties, such as increased transparency, reduced scattering of visible light, mechanical properties and barrier performance. Furthermore, there is a need for methods to control cation charge distribution within the interlayers of surface-modified filler, to facilitate control over the surface hydrophilic/lipophilic balance of the surface-modified filler.